Effects of hyperthermia on mouse L cells irradiated with fractionated X-rays

The responses to X-rays and radiosensitizing effects of post-treatments with heat were examined in the cells exposed to different doses of multifractionations. When cells were irradiated with daily doses of 2 Gy the radiation responses changed during multifractionated treatments, giving rise to two components in survival curves. The changes may be due to selective killing of radiosensitive cells or a cell progression through radioresistant phases during multifractionation irradiation as shown in analysis of the cell cycle. However, the survival curves became almost exponential when fractionation was performed with daily doses of 3-4 Gy. Radiosensitization by heat at 43 degrees C for 15 min was detectable in the cells after exposures to fractionated doses of more than 7 Gy with daily doses of 2 Gy, but not after exposures to daily doses of 3-4 Gy. Radiosensitization by heat at 43 degrees C for 30 min was manifest as disappearance of shoulder and increase of the slope in survival curves after irradiation with daily doses of 2 Gy. It was observed as an increase of slope in survival curves when cells were irradiated with daily doses of 3 or 4 Gy. Such differences in the radiosensitization may be due to a difference in the susceptibility to heat among the cells which had been exposed to the different doses and regimens of fractionated irradiation.